THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 65 



segment, and does not supply any portion of the neighbouring branchial 

 segments. The nerve-supply in Ammoccetes gives no countenance to 

 the view that the original unit was a branchial pouch, the two sides 

 of which each nerve supplied, but is strong evidence that the original 

 unit was a branchial appendage, which was supplied by a single 

 nerve with both motor and sensory fibres. 



Any observer having before him only this picture of the respiratory 

 chamber of Ammoccetes, upon which to base his view of a vertebrate 

 respiratory chamber, would naturally look upon the branchial unit of 

 a vertebrate as a gilled appendage projecting into the open cavity of 

 the anterior part of the alimentary canal or pharynx. This is not, 

 however, the usual conception. The branchial unit is ordinarily 

 described as a gill-pouch, which possesses two openings or slits, an 

 internal one into the lumen of the alimentary canal, and an external 

 one into the surrounding medium. This view is based upon embryo- 

 logical evidence of the following character : — 



The alimentary canal of all vertebrates forms a tube stretching 

 the whole length of the animal ; the anterior part of this tube 

 becomes pouched on each side at regular intervals, and the walls of 

 each pouch becoming folded form the respiratory surfaces or gills. 

 The openings of these separate pouches into the central lumen of the 

 gut form the internal gill-pouch openings ; the other extremity of 

 the pouch approaches the external surface of the animal, and finally 

 breaks through to form a series of external gill-pouch openings. 



From the mesoblastic tissue, between each gill-pouch, there is 

 formed a supporting cartilaginous bar, to which are attached a system 

 of branchial muscles, with their nerves and blood-vessels. These 

 cartilaginous bars, in all fishes above the Cyclostomata, form a 

 supporting framework for the internal gill-slit, so that the gills 

 are situated externally to them ; the more primitive arrangement is, 

 as already mentioned, a system of cartilaginous bars, extra-branchial 

 in position, so that the gills are situated internally to them. 



From this description of the mode of formation of the respiratory 

 apparatus in water-breathing vertebrates the conception has arisen 

 of the gill-pouch as the branchial unit, a conception which is 

 absolutely removed from all idea of a branchial unit such as is 

 found in an arthropod, viz. an appendage. 



This conception of spaces as units pervades the whole of embryo- 

 logy, and is the outcome of the gastrula theory— a theory which 



